In general, a Doherty power amplifier is actively studied as a technique for improving performance of a power amplifier in a backoff region.
The Doherty power amplifier is configured by connecting a carrier amplifier and a peaking amplifier in parallel using a quarter-wave transformer λ/4 line. Hence, as a power level increases, currents supplied from the peaking amplifier to the load increase. The Doherty power amplifier improves the efficiency by regulating load impedances of the carrier amplifier and the peaking amplifier.
According to the configuration, the Doherty power amplifier may include a symmetric 2-way Doherty power amplifier and an N-way Doherty power amplifier including (N−1)-ary peaking amplifiers.
The symmetric 2-way Doherty power amplifier cannot amplify a modulation signal of Peak to Average Power Ratio (PAPR) with a maximum efficiency because the backoff level is limited to 6 dB.
The N-way Doherty power amplifier can achieve higher efficiency of the modulation signal than the symmetric 2-way Doherty power amplifier, using the (N−1)-ary peaking amplifiers and phase delay lines. However, since the size rate of the peaking amplifier increases by integral multiples of the size of the carrier power amplifier, the backoff level of the N-way Doherty power amplifier is limited to 6 dB, 9.54 dB, and 12 dB as shown in FIG. 1. In addition, the N-way Doherty power amplifier has more input loss than the symmetric 2-way Doherty power amplifier.